EMF of galvanic cells

Electrode potentials (as well as values of the EMF of galvanic cells) depend on the composition of the electrolyte and other phases of variable composition. The electrode potential corresponds to the Galvani potential of the electrode-electrolyte interface, up to a constant term f =(Po + const. Introducing the concendation dependence of the chemical potential p into Eq. (3.21), we find that... 

Direct measurements of solute activity are based on studies of the equilibria in which a given substance is involved. The parameters of these equilibria (the distribution coefficients, equilibrium constants, and EMF of galvanic cells) are determined at different concentrations. Then these data are extrapolated to very low concentrations, where the activity coincides with concentration and the activity coefficient becomes unity. 

Potentiometry is used in the determination of various physicochemical quantities and for quantitative analysis based on measurements of the EMF of galvanic cells. By means of the potentiometric method it is possible to determine activity coefficients, pH values, dissociation constants and solubility products, the standard affinities of chemical reactions, in simple cases transport numbers, etc. In analytical chemistry, potentiometry is used for titrations or for direct determination of ion activities. 

The negative logarithm of hydrogen ion concentration was defined by Sorensen S as the pH.t Sorensen did not actually measure hydrogen ion concentrations, but something more nearly related to activities. He measured the emf of galvanic cells such as... 

Measurements of the emf of galvanic cells can be used to advantage to extract thermodynamic information concerning the characteristics of chemical reactions. As already stated, corresponding to the symbolic chemical reaction one may specify an equilibrium parameter Kx = Oy If cell can... 

In order to decide the suitability of a certain melt in technical practice, an in-depth knowledge of its physico-chemical properties is unavoidable. The present database of the properties of inorganic melts is relatively broad. Many properties are known, such as phase equilibria, enthalpies of fusion, heat capacities, density, electrical conductivity, viscosity, surface tension, emf of galvanic cells of many molten systems, the measurement of which was stimulated first by their technological application. 

When Nernst starteid work alongside Arrhenius in Ostwald s Leipzig laboratory in 1887, there was uncertainty as to the location and quantification of the electromotive force (emf) of galvanic cells. 

A problem with compiling a list of standard potentials is that we know only the overall emf of the cell, not the contribution of a single electrode. A voltmeter placed between the two electrodes of a galvanic cell measures the difference of their potentials, not the individual values. To provide numerical values for individual standard potentials, we arbitrarily set the standard potential of one particular electrode, the hydrogen electrode, equal to zero at all temperatures ... 

The EMF values of galvanic cells and the electrode potentials are usually determined isothermally, when all parts of the cell, particularly the two electrode-electrolyte interfaces, are at the same temperature. The EMF values will change when this temperature is varied. According to the well-known thermodynamic Gibbs-Helmholtz equation, which for electrochemical systems can be written as... 

Thus, through the example of the galvanic (Cu-Zn) cell, it has been shown that the emf of a cell is equal to the difference between the electrode potentials of the electrodes of which it is constructed. It may be recounted here that the thermodynamic text presented here sup-... 

Having introduced matters pertaining to the electrochemical series earlier, it is only relevant that an appraisal is given on some of its applications. The coverage hereunder describes different examples which include aspects of spontaneity of a galvanic cell reaction, feasibility of different species for reaction, criterion of choice of electrodes to form galvanic cells, sacrificial protection, cementation, concentration and tempera lure effects on emf of electrochemical cells, clues on chemical reaction, caution notes on the use of electrochemical series, and finally determination of equilibrium constants and solubility products. 

So far, a cell containing a single electrolyte solution has been considered (a galvanic cell without transport). When the two electrodes of the cell are immersed into different electrolyte solutions in the same solvent, separated by a liquid junction (see Section 2.5.3), this system is termed a galvanic cell with transport. The relationship for the EMF of this type of a cell is based on a balance of the Galvani potential differences. This approach yields a result similar to that obtained in the calculation of the EMF of a cell without transport, plus the liquid junction potential value A0L. Thus Eq. (3.1.66) assumes the form... 

The EMF of a galvanic cell is a thermodynamic equilibrium quatity. Thus, the potential of a cell must be measured under equilibrium conditions, i.e. without current flow. The measured EMF must be compensated by a known external potential difference. The measurement of the EMF of a cell is thus based on determination of a potential difference that exactly compensates the measured potential difference so that no current passes. This is easily achieved by the Poggendorf compensation method (see Fig. 3.13). 

With an understanding of the meaning and measurement of the difference of electrical potential, we can develop the thermodynamics of a galvanic cell. We choose a specific cell, but one in which many of the principles related to the obtaining of thermodynamic data from measurement of the electromotive forces (emf) of the cell are illustrated. The specific cell is depicted as... 

The simplest type of such a cell is a combination of two different metal electrodes which are immersed into solutions of their own salts. If the existence of the liquid junction potential at the interface of both solutions is not taken into account, or if this potential has been suppressed in a suitable manner, the EMF of the cell equals the total of the oxidation and reduction potentials at the electrodes. E. g. in a galvanic cell composed according the following formula... 

Sometimes the potentials are measured by the commutator method during which the electrolyzing current is suddenly interrupted and the value of the EMF of the cell shown by the brief reverse deflection of the voltmeter is quickly read. The voltmeter is connected in an electric circuit parallel with the electrodes. The deflection is caused by products accumulating at the electrodes in the course of electrolysis. The system acts for a short period as a galvanic cell. 

Derive formulas showing how ACp d and AVd may be determined from emf measurements of galvanic cells. 

E = EMF, or Electromotive Force, or Cell Potential (In the context of galvanic cells) also see below... 

When dissimilar metals or electrodes are immersed in an electrolytic solution with common ions, an electromotive force (EMF) develops between the electrodes. This is the principle behind formation and working of galvanic cells. The EMF is characteristic of the free energy change in ion exchange (i.e. the cell reaction). 

The general operation of galvanic cells is symbolized by the overall reaction v/A,- = 0, carried out only to an infinitesimal extent, which does not significantly alter the concentrations of chemical species or the emf. Advancement of... 

Electrons participating in the intercalation/deintercalation reaction (Equation (5.1)) can be represented by a current-producing system. Second, it is characteristic that the current-producing system reversibly operated by a self-driven (galvanic) cell (discharging the battery) performs the electrical useful work AG = —zFE (where E is the EMF of the cell), because electrical potential difference is spontaneously developed between two electrodes. By contrast, when the cell is short-circuited - that is, when the two electrodes are not separated from each other but are directly in electrical contact - electrons do not appear explicitly but rather participate in corrosion (or permeation in the case of solid electrolyte cells). They perform no electrical useful work because the two electrodes have the same electrical potential. 

The determination of pH is often performed by emf measurements of galvanic cells involving liquid junctions [69ROS], [73BAT], A common setup is a cell made up of a reference half cell (e.g. Ag(s)/AgCl(s) in a solution of constant chloride concentration), a salt bridge, the test solution, and a glass electrode (which encloses a solution of constant acidity and an internal reference half cell) ... 

---